A common objective in contemporary magnetic disk storage systems is to provide high track densities and fast average access times at lowest cost. Such requirements can be met with a quasi closed loop servo system which relies on external indicia for controlling "seek positioning" between tracks and a combination of external indicia and embedded servo signals for controlling "track following". Such a servo system requires some level of alignment between the external indicia and the recorded data tracks as defined by the embedded servo information. This alignment must remain throughout all environmental aggravations such as temperature, orientation, shock and vibration. This tends to complicate the mechanical design of the system, as well as its manufacturing and quality control processes, increasing the difficulty of obtaining high production yields of satisfactorily operating systems with expected operational longevity.
A problem recently noted is that a percentage of such systems which pass manufacturing and quality inspection tests, nevertheless may exhibit faulty operation after shipment to end users. As noted presently, in a large proportion of such systems, the problem is traceable to "post shipment" misalignment between the external indicia (e.g. graduations in an optical grid) and embedded servo signals which is beyond the correction capability of the servo system. The embedded signals are recorded during the manufacturing process in precise optimal alignment with the external indicia, and in normal use should remain aligned within a predefined tolerance range (whereby the offset of the head assembly from track center, at completion of a seek positioning operation, should permit sensing of the correct embedded signals and correspondingly correct track following operation). However, in unusual circumstances, this normal alignment condition may be lost and cause the system to detect an error condition which appears as a hard failure.
The present invention involves recognitions that such failures--and related inconveniences to end users and replacement or repair costs incurred by the manufacturer--are avoidable by rather simple modifications of the circuits associated with sensing of the external indicia and the system initialization process.